Ion Bombardment Effects on GaAs Photocathodes
Examining the impact of ion bombardment on bulk GaAs photocathodes with different surface-cleavage planes. Ion back-bombardment in photo-gun setups is a significant factor limiting the operating lifetime of photocathodes. Investigation includes H-ions implantation into GaAs, experimental apparatus details, and ion trajectory analysis from ion source to cathode.
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The effects of ion bombardment on bulk GaAs photocathodes with different surface-cleavage planes Wei Liu Shukui Zhang, Matt Poelker, Marcy Stutzman Jefferson Lab
Ion back-bombardment in photo-gun Ion back-bombardment is the dominant lifetime- limiting mechanism of modern DC high voltage photo-gun Ion back-bombardment: the residual gas within the cathode/anode gap ionized by the electron beam accelerated to and implanted into photocathode decrease the QE of photocathode (reduce the operating lifetime of photocathode) The electron ionization cross section change with the electron kinetic energy that ranges from 0 to 100 (even to 350) keV. Hydrogen is the dominant gas species inside a modern photo-gun QE map of photocathode in CEBAF J. Grames, PRST-AB 14, 043501 (2011) Electron impact ionization cross section for ?2 2
H-ions implantation into GaAs photocathode H-ions implant into the GaAs with different depth that depend on the H- ions energy When H-ions energy > 500 eV, they knock out the Ga or As atom leaving the vacancy in these positions Higher energy ions penetrate deeper into the GaAs and do more damage to the crystal structure compared to lower energy ions Hydrogen ion distribution within GaAs Vacancy distribution within GaAs 3
Experimental apparatus The chamber was baked at 200 C for 36 hours to achieve a high vacuum pressure: 10 11 Torr The H-ion was provided by a Pyrex glass dissociator in which the molecular hydrogen was dissociated using an RF- inductive discharge created by a 12 turn coils The H-ion was accelerated by the negative voltage on the cathode and implanted into the photocathode Schematic and photograph of the UHV test apparatus 4
Ion trajectory from ion source to cathode The ions have initial energy with different directions Two bias-voltage are supplied on the cathode independently, and a picoammeter connected the voltage power supply to monitor the ion current (dose) at the cathode. The ion trajectory at different bias-voltage are different -100 V on the cathode -10 kV on the cathode 5
Experimental Photocathode samples (100) (110) (111A) Dopant GaAs-Zn GaAs-Zn GaAs-Zn Orientation (100) 0.5 (110) 0.5 (111A) 0.5 The samples were cleaved into 15 15 mm squares and attached to a sample holder fixed by a tantalum cap Carrier concentration (a./c.c.) 1.0~1.3E19 1.3~1.4E19 1.1~1.14E19 Resistivity (ohm.cm) 6.6~7.7E-3 6.1~6.6E-3 7.17~7.38E-3 Mobility (cm2/v.s.) 74~80 71~74 77~78 Thickness ( m) 600~650 475~525 500~550 The specifications of three bulk GaAs samples with different surface-cleavage planes Three bulk GaAs samples with different surface- cleavage planes were used for this work This work serves to assess which cleave plane surface provides the highest QE and best resistance to ion implantation damage 13mm 6
Ion channeling for three GaAs surfaces Ion channeling: the positively charged particles (like the hydrogen ions) tend to follow the direction between two neighboring crystal planes The effective atomic number density for the (100), (110) and (111A) GaAs cleave planes are 8/?2, 4 2/?2, and 4 3/?2, respectively. Ion channeling would be enhanced for GaAs (110) which possesses the smallest atomic number density (i.e., the most open space between crystalline layers) Schematic of the GaAs crystal for three surface-cleavage planes, viewed at normal incidence. From left to right, (100), (110) and (111A). ? = 5.6535 7
Experiment process 1. Heat sample to 550 C for 1 hour and then cool to room temperature (RT) 2. Activate the photocathode by applying Cs and ??3 to the surface using a standard yo-yo activation protocol 3. QE was mapped using a low power green laser beam and QE vs wavelength was measured 4. Heat sample to 200 C for 30 minutes and then cool to RT 5. The sample was biased at either 100 or 10,000 V and exposed to H-ions of a prescribed dose 6. The sample was heated and re-activated multiple times to observe QE evolution 8
QE Vs wavelength for three samples The cleave planes (100) and (110), with equal amounts of Ga and As atoms at the surface, provided the highest QE, approximately 21% at the laser wavelength of 532 nm The cleave plane (111A), comprised of only Ga atoms at the surface, provided the lowest QE, approximately 19% at 532 nm QE versus laser wavelength for three bulk GaAs with different surface-cleavage planes, measured before ion implantation 9
Typical QE map of photocathode For 100 V ions: QE degradation was uniformly distributed across the sample For 10,000 V ions: QE degradation was focused on to a specific region of the sample Typical QE maps for bulk GaAs (111A) sample. (left) after third activation following implantation with 100 V H-ions. (right) after the third activation following implantation with 10,000 V H- ions 10
QE evolution for three samples Both 100 V and 10,000 V H-ions degrade the QE The QE degradation associated with 10,000 significantly greater than caused by 100 V ions Heating the sample to higher temperature and for long time restore QE For 100 V ions: QE could be completely restored by heating sample; For 10,000 V ions: only a fraction of the QE could be restored T ( C) 250 370 490 550 550 t (min) 20 30 60 60 60 V ions was QE (532 nm) versus heat cycles for bulk GaAs with three different surface cleave planes 11
QE ratio evolution for three samples The GaAs (110) - i.e., the sample with the most open space between atoms, and therefore the sample supporting the highest level of ion channeling - exhibited the highest QE ratio The GaAs (100) - i.e., the sample with the least open space between atoms, and therefore the sample supporting the lowest level of ion channeling - exhibited the lowest QE ratio QE ratio (532 nm) - samples implanted with 10,000 V H-ions to samples that were not exposed to H-ions for bulk GaAs with three different surface cleave planes T ( C) 250 370 490 550 550 t (min) 20 30 60 60 60 12
Summary Bulk GaAs was activated to QE near 20% at 532 nm. GaAs (100) provide the highest QE, and GaAs (111A) provide the lowest QE Both high and low energy H-ion implantation reduce QE Cleave plane (110) was the least sensitive to ion bombardment and cleave plane (100) was the most sensitive The best choice for a photo-gun using bulk GaAs would be cleave plane (110) Max QE (532 nm) Recovered QE (100 V) Recovered QE (10,000 V) Plane (100) 21.16 19.09 6.27 (110) 20.89 20.18 7.58 (111A) 18.67 18.4 6.34 13
